rod ratio vers eng acceleration

[L89 chevelle]

Originally Posted by Stan Weiss

The trend in drag racing has been to shorter rod. Way back the hot setup was a 4" bore 3" stoke 5.7" rod 302 CI 1.9 R/S. Small block Pro Stocker 4.06" bore 3.25" stoke 6.0" rod 337 CI 1.846 R/S. Small block Pro Stock Truck 4.19" bore 3.245" stoke 5.58" rod 357 CI 1.722 R/S. Each step along the RPM's went up and head flow has gone way up.

Great info. This is the stuff I was talking about. I bet the dyno didn't show any or very small but the on track showed bigger results.

[big block fiero]

I would really like to hear L/S ratio comparisons made in conjunction with different valve seat angles, rocker ratio's, and low lift flow changes that also make an engine exibit symptoms similar to these L/S ratio changes.

Where I am going with this is that these changes all alter the pressure recovery dynamics of the mixture flow across the valve seats. Putting it another way, if the mixture velocity is greater and more consistent at low lift even when the piston may not be moving enough air near tdc, then, the fuel will stay better vaporized. Try to realize that when the intake valve/port shuts and is then waiting for the next intake cycle, the velocity has also stopped. So then what do you think happens to the fuel that was suspended in the air stream and where does its enertia make it stop, on the next opening cycle just as the valve opens, the seat kind of drools a little liquid fuel at first.

Say you have long rods but also have 60 degree valve seats. Less air would be moving near tdc due to long rods and reduced piston speed, but the 60 degree seats will create more low lift flow velocity because they move less air thru a smaller opening at low lift -------- conversely to this now say youve got a short rod engine that picks up mixture velocity sooner after the overlap induced velocity has deminished. Now this motor could run 30 degree seats, higher ratio rockers, ect, with potentially less mixture related problems because the pressure recovery dynamics aren't as bad with the shorter rods.

Now---- to expand on this, what if you,ve got a super vizard carborator that vaporizes fuel better? Are you then as concern about good pressure recovery dynamics that vaporizes fuel better during low lift flow? What if you are running propane or a diesel? What L/S ratio would be desired then?

Next issue----is a particular L/S ratio or power stroke piston movement better suited to the combustion characteristics of a quenchless combustion chamber or a less quench area dished or nascar style dished piston?

Matt kneen, mpls mn.

[Builder]

Originally Posted by Stan Weiss

Unless you are building for a ci/weight class or have $$$$$ to spend the best thing is to get the pin and ring package / piston the best they can be and let the rod length fall where it will.

The question I have is what is the minimum compression height?

My application is not maximum effort but to get the most I can with durability (50k miles) and absolute reliability.

Builder

[1989GTA]

The main reason I went with the longer rod was the rotating assembly weight came down. On my SBC I was able to go with a 6.125" rod and with the Mahle pistons still keep the piston pin below the oil ring. This is mainly for a street car with occasional strip time.

[Furby]

I kinda go along with Cammer's reply. The rods job is to keep the piston conected to the crank at all times. Once that is acomplished, look elsewhere for power gains.

It would be tuff to ever verify a shorter/longer rod helped, or hurt.

How could you change rod lenght, and keep EVERYTHING else the same?

Piston weight/rod weight/valve to piston clearance/etc.

A rod lenght change requires a different piston, will it be EXACTLY the same, with a different pin height?


Furby

[302tt]

There seems to be a lot of trade-off's with this rod ratio theory and I just wanted to summarize a few of them to make sure I am clear on it all and how that relates to power...

RR= rod ratio, so a lower RR could be a stroker where stroked increase more than the rod length.

Longer rod = lower acceleration = higher rpm and more power
Longer rod = lighter piston due to less compression height = higher rpm, more power
Longer rod = longer dwell at TDC = increased detonation risk so less timing/power
Longer rod = increased thrust angle = more friction so less power

thats all i have so far...

But, here is a calc I did for a 302 Ford example;
302ci V8 with 4" bore, 3" stroke and 5.09" rod length = 1.59 rod ratio (50.4 g acceleration)
347ci V8 with 4" bore, 3.4" stroke and 5.4" rod length = 1.59 rod ratio (58 g acceleration)

To have the same acceleration the 3" stroke engine could rev to 6435 rpm which could net 7% more power (for the same torque) but the 347 engine would gain 15% power at 6000 rpm assuming torque is proportional to cid. In this example the stroker wins with the poorer (lower) rod ratio. Also the 347 will probably have a lighter piston so it can rev past 6000 rpm for the same tensile load on the rod bolts.

But, friction goes up with the smaller rod angle so the 347 will not last as long and will have higher FMEP! It's a world of trade-off's

[1989GTA]

"Longer rod = increased thrust angle = more friction so less power"

Actually I think you have that backwards.

"Longer rod=decreased thrust angle = less friction so more power"

[Furby]

Originally Posted by 302tt

Longer rod = lower acceleration = higher rpm and more power

This is true only at the top of the stroke. At the bottom of the stroke deccel/accell will be higher with a longer rod. The average piston speed will be the same with both rod lenghts. But peak piston speed will be slightly higher with a shorter rod.

Using your 3" stroke at 6500rpm @ 5" rod will have a peak piston speed of 5331fpm @ 74.5* ATDC.
The same 3" stroke at 6500rpm with a 6" rod will have a peak piston speed of 5263fpm @ 76.6* ATDC.

But which one would make the most power I wouldn't begin to guess.

Originally Posted by 302tt

Longer rod = lighter piston due to less compression height = higher rpm, more power

A lighter piston should help power, but I don't think it will move the power to a higher rpm.

Originally Posted by 302tt

Longer rod = longer dwell at TDC = increased detonation risk so less timing/power

I think other factors besides dwell time at tdc have a bigger affect on detonation.
Like chamber shape/fuel used/chamber temp/rpm etc.

Originally Posted by 302tt

Longer rod = increased thrust angle = more friction so less power

I think a shorter rod has an increased thrust angle, and therory, more friction.


I think the only time a rod has a significant effect on power, is when it failed to keep the piston connected to the crank.

Furby

[MrWOT]

Longer rods will benefit N/A engines provided you have enough octane to survive any low rpm use. Force fed engines tend to want shorter rods. It all depends on the burn cycle, lots on the fuel you're using, ignition, chamber dynamics at certain piston positions, and a slew of other things. The last turbo engine I made used very short rods, under 1.5 r/s, ran 18psi and lived from 2500-6500 on 91 octane, had excellent output, no signs of detonation from knock sensors. That engine died to a split bore, which I suspect was more due to a casting flaw. But increased side loading from the shorter rods is a thought too..

[rskrause]

The lighter piston and lower overall piston+rod weight in a long rod combo always seemed important to me as something which is going to reduce tensile loads on the rods and bolts and also allow the engine to accelerate faster (less reciprocating mass). Hard to see that it would have a big impact on hp per se though. The rod ratios in F1 are much higher then 2:1. I believe I read up to 2.8:1 somewhere. Gotta be a reason for that. Their R&D budget exceeds any other race series by orders of magnitude.

Richard